Recently, in a rapidly-developing mobile communication system (for example, Personal Handyphone System: hereinafter, referred to as “PHS”), in communication between a radio base station system (a base station) and a mobile terminal (a terminal), a scheme for separating and extracting a reception signal from a specific terminal with an adaptive array processing on a side of a base station, among a plurality of terminals that have established spatial multiple access to the base station, has been proposed.
The adaptive array processing is a well-known processing, in which, based on signals received from a plurality of terminals by an array antenna of the base station, a reception weight vector consisting of weights for respective antennas constituting the array antenna is calculated and adaptively controlled, thereby accurately extracting a signal from a specific terminal.
In the base station, a reception weight vector calculator calculating such a reception weight vector for each symbol in the reception signal is provided. The reception weight vector calculator performs a processing to converge the reception weight vector so as to reduce a mean square of an error between the sum of complex multiplication of the reception signal by a calculated reception weight vector and a known reference signal, that is, the adaptive array processing to converge reception directivity from the specific terminal.
In the adaptive array processing, the reception weight vector is adaptively converged in accordance with time and fluctuation of a property of a propagation path for a signal radio wave, so that an interfering component or noise is eliminated from the reception signal, to extract the reception signal from the specific terminal.
On the other hand, in the base station, a transmission weight vector calculator calculating a transmission weight vector for each symbol in a transmission signal is provided. The transmission signal weighted with the transmission weight vector calculated by the transmission weight vector calculator is transmitted with the same array antenna as in reception.
Here, if the transmission weight vector calculator duplicates the reception weight vector as it is for use as the transmission weight vector, the transmission signal is weighted to target the same, specific terminal as in reception. In such a case, a transmitted radio wave signal is emitted as if it had transmission directivity to that terminal.
In contrast, there is a time interval from when the radio wave is transmitted from the specific terminal to the base station through an uplink until when the radio wave is transmitted from the base station to the terminal through a downlink. When the reception weight vector obtained in the uplink is used as it is in the downlink as the transmission weight vector as described above, and if a moving speed of the terminal is not negligible, there will be an error between a direction of emission of the radio wave from the base station and a direction in which the terminal is actually present. This will lead to deterioration of transmission directivity from the base station in the downlink.
As a method of calculating a transmission weight vector for a downlink, taking into account fluctuation in a propagation environment (fading) mainly due to travel of the terminal, a method with which a transmission response (coefficient) vector in the downlink is estimated with a extrapolation processing using a reception response (coefficient) vector obtained in the uplink, and the transmission weight vector is calculated based on the estimated transmission response vector, has been proposed.
In other words, the reception response vector is measured at a plurality of timings in the uplink in a frame, and based on the measurement value, a time waveform of the reception response vector is subjected to multiple-order extrapolation (for example, first-order extrapolation or second-order extrapolation). Then, the reception response vector at an arbitrary transmission timing in the downlink is estimated, and this will serve as the transmission response vector. The transmission weight vector is thus calculated.
With such a method, in a state where a Doppler frequency (hereinafter, represented as “FD”) in proportion to the moving speed of the terminal (representing a degree of fading) is approximately up to 40 Hz, spatial multiple access of terminals of two users to the base station can be maintained. The fact that spatial multiple access can be maintained up to FD=40 Hz is disclosed, for example, in “Weight estimation for down-link null forming TDD/SDMA system,” Technical Report of IEICE, (CS99-44, RCS99-36, June, 1999).
With the method using the extrapolation processing as described above, however, if the moving speed of the terminal is increased, and the time waveform of the reception response vector abruptly fluctuates with a cycle being short, an extrapolation result will considerably deviate from an actual time waveform of the reception response vector, and an extrapolation error will be large.
In particular, if the Doppler frequency representing the moving speed of the terminal exceeds FD=40 Hz, spatial multiple access can no longer be maintained.
The present invention was made to solve the above-described problems. An object of the present invention is to provide a radio base station system, as well as a method and a program for controlling transmission directivity, with which a transmission response vector in a downlink can accurately be estimated, and transmission directivity can sufficiently be controlled, even if the moving speed of the terminal is increased, and the Doppler frequency is raised.